Oxide semiconductors such as InGaZnO have been mainly applied to devices for displays. Application of oxide semiconductors to thin film field effect transistors formed on a wiring layer of a large scale integrated circuit (LSI) is expected. The thin film field effect transistors formed for LSIs are required to decrease their size.
Therefore, a top gate type structure formed on a conventional silicon substrate that can apply self-alignment process is desirable. However, formation of a diffusion layer by impurity implantation and the like cannot be performed on the oxide semiconductor transistors, which is different from conventional silicon transistors. Therefore, when the top gate type structure is employed, large parasitic resistance of source/drain regions becomes a problem.